Water purification plants clarify water for redistribution in the public supply, and in so doing they obtain fluid sludge containing mineral substances and vegetable or organic fibres. Similar sludge is obtained in the paper making industry and in other industries.
These types of sludge are generally not very concentrated and according to the mineral substances and the type of fibres contained, the concentration of the sludge is generally very small, i.e. a few grams per liter, so their handling is difficult.
Sludge in this state cannot be handled by conventional means of transport: trucks, conveyor belts, etc. It must be disposed in sewage tanks for settling to concentrate them. Then after a more or less long period of time they can finally be drained.
In some cases industries and urban water purifying installations use methods for accelerating the final draining of the sludge. These conventional methods, such as draining on filters or centrifugation, generally require a large investment and entail high operational costs.
Before describing the method which is the subject of this patent application, it would be useful to mention a few general details on the expression "sludge" as used in this specification: it refers to sludge containing dry materials formed by finely divided mineral materials and by fibres in more or less equal parts.
When the concentration of the dry substances is less than 1 g/l, the terms "dirty" or "charged" water or liquor are used.
When the concentration exceeds a few grams per liter, the term "fluid decantation sludge" is used; at a few tens of grams per liter, the sludge is pasty. When the concentration of solid substances reaches 250 to 300 g/l, a solid mass is obtained.
The present invention is directed to an improvement to the sludge compacting device disclosed in U.S. Pat. No. 3,923,652.
The main features described in this patent application are reviewed herebelow.
When it is required to decant fine dry mineral or organic substances rapidly, it is advantageous to insert in the charged liquid small quantities of materials, such as, mineral salts, organic or synthetic floculation agents, so as to concentrate the dispersed dry substances in the form of agglomerations, much larger than the unitary substances so as to speed up decanting.
By forming these flakes, a primary floculation phenomenon is started and small, thin flakes which are not very dense and not very solid are formed. If suitable floculation substances are again added, a second floculation, called the secondary floculation, is obtained with larger and more solid flakes.
After having effected the primary and secondary floculation described hereinabove, the invention described in the said patent consists in a controlled stirring of the sludge in a tank having a shape suitable for ensuring sludge settling. The water or the liquid obtained during the settling being removed into the top part of the device, the clods of settled sludge are removed by suitable means at the bottom of the device.
This controlled stirring of the sludge is effected by slow mechanical stirring throughout the sludge using judiciously-inclined perforated blades in the tank containing the sludge to cause a mechanical primary clumping of the flakes. The liquid extracted from the flakes rises to the top of the device, whereas the clumped flakes are driven to the bottom.
The clumped flakes coming from the top part to the bottom of the tank are driven by a conical screw having one or several blades rotating slowly in a conical recess narrowing downwardly and constituting the sludge output orifice.
This disposition enables the completion of the settling effect started in the top part of the device. Indeed, due to the shape of the screw, the discharge of the sludge decreases the nearer the extraction orifice of the device is approached so as to send back upwards the interstitial liquid which is still between the flakes and to make the latter come out in a compact mass through the extraction orifice.
Such an arrangement operates well in industry as described in the patent, but proves to be too effective for mineral substances carrying fibrous substances.
Indeed, such sludges forms flakes which, due to tangling of the fibres in the flakes, become very solid in the top stage of the device. These flakes are then made so compact by the extraction screw that the extraction discharge decreases too rapidly and the dry substance becomes solid and in some cases it is not possible to extract it from the bottom of the device.